Hydrothermal synthesis of corrensite; a study of the transformation of saponite to corrensite

State University of New York at Binghamton, Department of Geological Sciences, New York, NY, United States

Hydrothermal synthesis experiments were conducted to study the transition from smectite to corrensite. A mixture of oxides with the bulk composition of corrensite--Na (sub 0.4) (Si (sub 6.4) Al (sub 1.6) )(Mg (sub 7.8) Al (sub 1.2) ) - O ₂₀ (OH) ₁₀ --was sealed in platinum capsules with 29-37 wt. % water. One set of samples was treated in cold-seal vessels at 500 degrees C and 2kbar for durations of 2, 3, 6, 12, and 24 h; the other set was treated at 350 degrees C and 2kbar for periods of 12 to 89 d. X-ray diffraction patterns (XRD) of oriented aggregates from treated products were obtained from ethylene glycol-solvated and air-dried preparations. Samples were also heated to 350 degrees C either in a calibrated muffle furnace, removed and quickly places in a nitrogen filled chamber on the diffractometer, or were heated at 350 degrees C by using a calibrated heating stage mounted on the diffractometer. Initial mineral assemblages at both temperatures contained only saponite and serpentine. In experiments at 500 degrees C, saponite transformed to corrensite within 6 h; in experiments at 350 degrees C, the transformation occurred as early as 22 d. Increased experiment times at both temperatures produced increasing amounts of well-crystallized corrensite, as indicated by several well-defined XRD peaks. No evidence of randomly interstratified chlorite-smectite (C-S) precursor to corrensite was found. The identification of pure smectite, as opposed to highly-expanded randomly interstratified C-S, was possible only when clays were dehydrated on a heating stage on the diffractometer. These results call for a new examination of hydrothermally-altered basalt that has been reported to contain randomly interstratified C-S as an intermediate step in the reaction of smectite to corrensite or chlorite. These results also strengthen the view held by increasing numbers of investigators that corrensite should be regarded as a single phase, not as a mixed-layered silicate.

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